1. Field of the Invention
The present invention relates to an electro-acoustic conversion film used in a digital acoustic device such as a digital speaker, and to a digital speaker using the same.
2. Description of the Related Art
A digital speaker is a speaker to which a digital signal (pulse) is directly input so as to obtain a conventional analog acoustic output. Here, an analog signal is sampled at regular intervals using a “sampling theorem”, and a pulse having a sample value is applied to the speaker and subjected to digital/analog conversion (D/A conversion).
An electrodynamic speaker using a permanent magnet essentially has a D/A conversion function, and a large number of digital speakers may be considered using the function.
In order to perform the D/A conversion, it is necessary to have an acoustic output value corresponding to all of sample values of an input pulse. Specifically, it is necessary to perform weighting of output up to 2N-1 for a speaker for a signal system having a pulse with a maximum of N bits. For example, in an 8-bit signal system, a weighted output change is required from a minimum value 20 (=1) to a maximum value 27 (=128).
For this weighting method, “multi-unit scheme” and a “multi-voice coil scheme” are known.
The “multi-unit scheme” is a scheme of performing acoustic synthesis in a space using a total of n units each having a weight corresponding to one bit. On the other hand, the “multi-voice coil scheme” is a scheme of performing weighting in a voice coil execution winding length Ω.
However, in the “multi-unit scheme”, there is a problem, for example, in that a larger number of electrodynamic speakers are required as the number of bits increases. Moreover, in the “multi-voice coil system”, there is a problem, for example, in that a loss of magnetic energy increases with a decrease in space factor as the number of bits increases.
On the other hand, a digital speaker using a piezoelectric speaker different from an electrodynamic speaker has been proposed.
For example, JP1984-95796A (JP-S59-95796A) and JP1984-95799A (JP-S59-95799A) describe a digital speaker including a vibration plate formed of a piezoelectric element between a pair of flat electrodes, in which one of the flat electrodes includes a plurality of unit electrodes radially divided at substantially a constant angle, and the unit electrodes are grouped so that an area is proportional to a weight of each bit digit of a digital signal.
Further, the Journal of the Institute of Electrostatics Japan (Vol. 11 @ No. 3, 1987) pp. 150-157 discloses a digital speaker in which an area of an electrode deposited on a surface of a polymer piezoelectric material is concentrically divided into seven such that it increases in size twofold from a center to the outside.